Improving/extending QML type system

Notes

- optionals are actually sum types of void and SomeType - actual sum types? QVariant converters are evil because - extend QMetaType to specify sum types and underlying types

 - possibly expressed as std::variant

- allow that as types in QML to allow restrictions on what can be assigned to a property - Something similar with function types

 - possibly expressed as std::function
 - QMetaMethod?
 - member functions!

- Interface types? We have sort of half interfaces in QML already. Let's not invent another one.

In QML: optional/sum unwrapping implicit or explicit? - z ?? "red" - z as color - There will be lots of corner cases.

Fabian will do some prototyping and post on mailing list.